The invention relates to a circuit arrangement for demodulating an information signal which is amplitude-modulated on a carrier wave.
A circuit arrangement having two cross-coupled transistor differential amplifiers, i.e. four transistors, is known from DE-OS 22 62 580. Therein each transistor has an electrode in common with each of the three other transistors. For example, a first transistor has the emitter terminal in common with a second transistor, the collector terminal in common with a third transistor and the base terminal in common with a fourth transistor. The interconnected collector terminals of the first and the third transistor are connected to a positive operating voltage via a first resistor from which the output voltage is derived. The interconnected collector terminals of the second and the fourth transistor are also connected to the operating voltage via a second resistor. The interconnected emitters of the first two transistors are connected to the collector terminal of a fifth transistor whose emitter terminal is connected to ground via a third resistor and to whose base terminal a positive direct voltage is applied, for which reason the fifth transistor is also referred to as DC transistor. In the same way a sixth transistor with a fourth resistor is arranged as an emitter resistor in the common emitter lead of the third and the fourth transistor, which emitter resistor has the same resistance as the third resistor. The sum of a signal voltage and a direct voltage is applied to the base terminal of this sixth transistor, while the last-mentioned direct voltage is equally large as the direct voltage at the base terminal of the fifth transistor. The sixth transistor is therefore also referred to as signal transistor. By means of the direct control voltage applied between the base terminals of the first and the fourth transistor on the one hand and the second and the third transistor on the other hand, it is possible to control the amplification of the signal applied to the base terminal of the sixth transistor.
Such a circuit arrangement may also be used for demodulating an information signal which is amplitude-modulated on a carrier wave, the modulated signal being applied to the base terminal of the sixth transistor or between the base terminals of the fifth and the sixth transistor. The two cross-coupled transistor differential amplifiers are controlled by means of the carrier wave or a square-wave at the frequency of the carrier wave as a "control direct voltage".
This circuit arrangement has the drawback that manufacturing tolerances in the manufacture of the resistors and transistors used, particularly also when manufactured as an integrated circuit on a semiconductor body, viz. deviations of the relative resistances and the transistor parameters, lead to a DC offset of the cross-coupled transistor differential amplifiers which may cause interference frequencies at the output. These errors lead to an unwanted distortion of the useful signal to be processed.